The invention relates to a shuttle for double-stitch sewing machines with horizontally arranged shuttle shaft according to the preamble of claim 1.
A shuttle corresponding to the preamble of claim 1 has been known from European Patent Application EP 0 489 980 A1.
In the case of that shuttle, the upper part of the bobbin case is friction-locked in the lower part of the bobbin case by a permanent magnet.
This arrangement results in a simple design and simplifies the operation of exchanging the bobbin.
On the external end face of the bobbin case removal means and/or thread-tightening means are provided in a known fashion.
However, no possibility of increasing the thread-holding capacity is disclosed, and the removal of the upper part of the bobbin case has not been generally improved, either.
In order to increase the thread-holding capacity of double-stitch shuttles, without applying greater changes to the sewing machine, a number of different solutions have already been proposed. The shuttle from Durkopp-Adler, part No. 271 2091, has a greater thread supply, and another shuttle with greater thread supply is known from German utility model DE 8234141 U1.
Both shuttles can be used in existing sewing machines without substantial changes. The increase of the thread-holding capacity of the shuttle is achieved in these cases substantially by an increase of the volume of the bobbin case. Consequently, these shuttles pull a greater length of needle thread below the needle plate to form the needle thread loop. This is disadvantageous under technical aspects because it subjects the needle thread to increased stress and because the thread take-up characteristics are no longer optimally balanced.
Especially when sewing at high speeds, this may lead to sewing problems.
For removing and locking the upper part of the bobbin case, both before-mentioned shuttles make use of a conventional locking and removal mechanism arranged on the external end plate of the upper part of the bobbin case.
This locking and removal mechanism has for many decades proved its value with shuttles having a horizontally arranged shuttle shaft.
For removing the upper part of the bobbin case, a tilting lever is tilted using the finger nails, which has the effect that a slide releases the form-locking engagement of the upper part of the bobbin case in the lower part of the bobbin case, while fixing at the same time the bobbin in the upper part of the bobbin case. By gripping the tilting lever between the forefinger and the thumb, the upper part of the bobbin case can then be axially withdrawn from the lower part of the bobbin case, together with the bobbin.
It is a disadvantage of this mechanism that it requires relatively much space, which has a negative influence on the needle thread demand of the shuttle.
In addition, tilting the tilting lever with a finger nail is a relatively complicated and uncomfortable operation.
From DE 29 53 743 A1 a shuttle has been known where the thread supply is said to be increased by having the bobbin placed directly in a single-part bobbin case where it is retained by magnetic force. Thus, the volume of the omitted upper part of the bobbin case is available for increasing the thread-holding capacity of the shuttle. The necessary thread tension is said to be produced in this case by friction between the bobbin and the bottom of the bobbin case.
For removing the bobbin, a slot is provided in the bobbin case so that the bobbin can be pulled off with the finger nails.
This shuttle has never been successful in practice, because the thread tension cannot be kept constant. At constant torque, which is required to rotate the bobbin, the thread tension is almost four times higher when the bobbin is almost empty than when it is full. In addition, the way of removing the bobbin is extremely awkward for the operator.